Bar Code Scanner

Background

Many different types of bar code scanning machines exist, but they all
work on the same fundamental principles. They all use the intensity of
light reflected from a series of black and white stripes to tell a
computer what code it is seeing. White stripes reflect light very well,
while black stripes reflect hardly any light at all. The bar code scanner
shines light sequentially across a bar code, simultaneously detecting and
recording the pattern of reflected and non-reflected light. The scanner
then translates this pattern into an electrical signal that the computer
can understand. All scanners must include computer software to interpret
the bar code once it's been entered. This simple principle has
transformed the way we are able to manipulate data and the way in which
many businesses handle recordkeeping.

Bar code scanning emerged in the early 1970s as a way to improve the speed
and accuracy of data entry into computers. Businesses were just beginning
to exploit computer tracking of stock and billing. The challenge was to
find a quick, efficient, and relatively fool-proof method of record entry
for companies (for example warehouses or mail order companies) that
maintain a small stock of high volume items. The use of bar codes enabled
clerks to keep track of every item they sold, shipped or packed without a
tedious and error-prone keyboard data entry process. Bar coding caught on
quickly in clothing stores, manufacturing plants (such as car makers),
airline baggage checks, libraries, and, of course, supermarkets. The
supermarket scanners which are commonplace today are known as
point-of-sale scanners, since the scanning is done when merchandise is
purchased; point-of-sale scanning is perhaps the most challenging bar code
scanning application in use today. Supermarket scanners represent the most
advanced design of the various types of bar code scanners, because of the
particular difficulties associated with reading bar codes on oddly shaped
items or items that may be dirty, wet, or fragile.

The first scanners required human action to do the scanning and used very
simple light sources. The most common was the wand, which is still popular
because it is inexpensive and reliable. Wand scanners require placing the
end of the scanner against the code, because the light source they use is
only narrow (focused) enough to distinguish between bars and stripes right
at the wand tip. If the labeled products are oddly shaped or dirty, this
method is impractical if not impossible.

To make a scanner that works without touching the code requires a light
source that will remain in a narrow, bright beam over longer
distances—the best source is a laser. Using a laser beam, the code
can be held several inches or more from the scanner, and the actual
scanning action can then take place inside the scanner. Rotating,
motor-driven mirror assemblies, developed in the mid-1970s, allowed laser
light to be swept over a surface so the user didn't need to move
the scanner or the code; this technology improved scanner reliability and
code reading speed.

Later, holograms were chosen to replace mirrors, since they can act just
like a mirror but are lightweight and can be motorized more easily. A
hologram is a photographic image that behaves like a three-dimensional
object
when struck by light of the correct wavelength. A hologram is created by
shining a laser beam split into two parts onto a glass or plastic plate
coated with a photographic emulsion. Whereas the previous generation of
scanners worked by rotating a mirror assembly, holographic scanners
operate by spinning a disk with one or more holograms recorded on it.

Researchers at IBM and NEC simultaneously developed holographic
point-of-sale scanners in 1980. Holographic scanning was chosen not only
because the hologram disks could be spun more easily than mirror
assemblies, but also because a single disk could reflect light in many
different directions, by incorporating different hologram areas on the
same disk. This helped to solve the problem of bar code positioning; that
is, codes no longer needed to directly face the scan window. Modern bar
code scanners will scan in many different directions and angles hundreds
of times each second. If you look at the surface of a scanner in the
checkout lane, you will see lots of criss-crossed lines of light; this
pattern was chosen as the most reliable and least demanding on particular
package orientation.

Raw Materials

A holographic bar code scanner consists of an assembly of preformed parts.
The laser—a small glass tube filled with gas and a small power
supply to generate a laser beam—is usually a helium neon (HeNe)
laser. In other words, the gas tube is filled with helium and neon gases,
which produce a red light. Red light is easiest to detect, and
HeNe's are less expensive than other kinds of lasers. They are much
smaller versions of the types of lasers used in light shows or
discotheques.

Lenses and mirrors in the optical assembly are made of highly polished
glass or plastic, which is sometimes coated to make it more or less
reflective at the red wavelength of light being used. The light detection
system is a photodiode—a semiconductor part that conducts
electrical current when light shines on it, and no current when no light
is present; silicon or germanium photodiodes are the two types of
photodiodes most commonly used.

The housing consists of a sturdy case, usually made of
stainless steel,
and an optical window that can be glass or a very resilient plastic. The
window material must have good optical and mechanical properties; that is,
it must remain transparent but must also seal the scanner from the air, so
no dirt or dust gets inside and blocks the light or the light detector.
Defects in the window can cause light to be transmitted at an
unpredictable angle or not at all; both scenarios affect the accuracy of
the scanner.

The holographic disks are made of a substance called
dichromated gelatin
(DCG) sealed between two plastic disks. DCG is a light-sensitive chemical
used to record laser images, much like photographic film records light. It
was developed by Dow Chemical and Polaroid for their own holographic work,
and it is sold in liquid form so that it can be coated onto a variety of
surfaces. DCG holograms are common in holographic jewelry (pendents, watch
faces, etc.) and in the holographic spinner disks sold in toy stores. DCG
will lose a recorded image if it is left in the open air, which is why it
must be sealed between two layers of plastic.

The spinning motor drive that turns the disk is a small electric cylinder
with a central spinning shaft, similar to the kind available in an erector
set. The shaft is attached to the center of the hologram disk, so that
when the motor is turned on, the disk spins.

Design

Bar code scanners require a team of designers to produce the completed
assembly. First, a laser recording engineer designs the hologram disk.
There are a number of important features to be considered in this design.
For instance, the disk must reflect the majority of light that hits it
(high efficiency), it must not distort the light so that the reflected
beam remains narrow, and it must reflect light in the chosen scan pattern
while it is spinning. Also, the scan pattern must maximize the number of
readable orientations at which a bar code can be passed over the scan
window and still be read.

The finished disk consists of many different holograms recorded in wedges
on the same disk. Each wedge reflects light at a different angle. As the
disk spins, the light is scanned in a line. The orientation of the lines
changes
from wedge to wedge. The hologram designer also specifies the exact power
of laser to be used, a choice based on longevity, efficiency and safety to
the user.

After the hologram disk is designed, an optical engineer designs the
placement of the laser and hologram disk, specifies any lenses or mirrors
required to steer the light in the right direction, and designs the
detection system so that light reflected from a bar code can be read
efficiently and reliably. The designer must optimize the scanner's
optical throw,
defined as the furthest distance an object can be held away from the
scanner window and still be read correctly. It is the job of the optical
designer to consider how best to fit the components into the smallest
space, with the smallest weight and expense, while still placing the
window at a convenient angle for normal use. For example, a supermarket
scanner must have the window facing up on the checkout stand, even though
it may be more convenient to put the spinning disk sideways inside the
box. Additional mirrors can allow both of these constraints to be met.

An electrical engineer determines the best method of interpreting the
electrical signals coming from the photodetector. Electrically, the
signals must be received and interpreted as a sequence of ON signals,
(light reflected from a white bar), and OFF signals, (no light reflected
from a black bar). The resulting pattern is then converted by a computer
into the product information the pattern represents. A computer programmer
may be employed to design the computer software that will translate the
code into product information, but the job of correctly interpreting the
ON/OFF pattern is left to the electrical engineer.

The Manufacturing
Process

After all of the components have been designed, they are ready to be made
and assembled. The hologram disk is generally manufactured in-house, while
the other components—lenses, mirrors, and laser—are usually
purchased from other manufacturers. The various parts are then assembled
and tested.

Hologram disk

1 The first step in the manufacturing process is to mass produce the
hologram disk. This disk is replicated from a master hologram. All the
disks, master and reproductions, are sandwiches made of plastic
"bread" with DCG filling. Master disks are made in
sections, one wedge for each different reflection angle required in the
final disk. A typical point-of-sale scanner will have between 7 and 16
wedges on a single disk. Holographic recording is done with two laser
beams that intersect at the surface of the DCG sandwich, creating the
holographic pattern. Adjusting the angle at which the two beams meet
will change the reflective properties of each hologram. Each wedge
created in this way will act like a mirror that is turned in a different
direction.

2 Once all the required wedges have been recorded, they are assembled
and glued down on a single transparent plate, which can then be
replicated. The glue used has optical properties that will not distort
the hologram image, such as glycerin-based adhesives will. There are
many ways to replicate a hologram, but the most common for DCG holograms
is optical replication. The master disk is placed close to, but not
touching, a blank DCG sandwich disk, and a single laser beam is used to
illuminate the master from behind. This transfers the pattern onto the
blank.

Lenses, mirrors, laser

3 Other components—lenses, mirrors, laser, etc.—are
usually purchased from an outside manufacturer. Lens, mirror and scan
window properties are specified during the design process. The
manufacturer tests all of these components as they arrive to confirm
that they meet specification. Motors and lasers are tested for proper
operation, and some are lifetime tested to make sure that the bar code
scanner will not fail within a reasonable period of time.

Housing

4 Housing can be purchased from a metal job shop, or it can be
fabricated by the manufacturer. The size and exact shape of the box is
specified in design, and manufacturing converts those specifications
into realizable sketches. The parts are machined, assembled and tested
for strength and durability.

In a bar code scanner, a laser beam is directed toward an item with a
black and white bar code symbol. The light is reflected back and
recorded on a spinning holographic disk. A photodetector then converts
this light into an electrical signal that can be read by a computer.
The spinning disk consists of a chemical substance, DCG, sandwiched
between two plastic disks. A typical holographic disk contains between
7 and 12 wedges, each of which reflects light at a different angle. To
make these disks, a disk master—comprising the various wedges
glued onto a single transparent plate—is first prepared. Next,
a single laser beam illuminates the master from behind, transfering
the pattern onto a blank DCG disk placed next to (but not touching)
the master.

Final assembly

5 Finally, the hologram disk is assembled with the spinning motor drive
and tested. Scanning pattern, direction, and speed are all examined. The
spinning disk is then assembled with the optical system (the laser and
mirrors). Placement of the laser often depends on space considerations:
the laser can be aimed directly at the spinning disk, or at a mirror
that guides the beam to the disk, if this makes the package smaller.

6 The disk and optical system are tested as a unit. When the assembly
passes inspection, it is mounted permanently inside the housing and
sealed with the scanning window.

Quality Control

There are several stages to quality control in bar code scanner
manufacturing. To begin with, there are several test criteria that are
defined within the bar code industry and that must be specified by all
manufacturers. These include:

First Pass Read Rate (FPRR)—the percentage of time that a code
can be read the first time it passes the scan window

Rejection Rate—the number of scans per million which simply
won't be read

Read Velocity—the range of speeds with which a code may be passed
over the surface of a scanner

These properties will relate to the optical, electrical and mechanical
properties of the scanner. Mechanically, scanners are run for several days
(and some select units will be pulled from production for longer lifetime
tests—up to several years) to insure that the motor will continue
to turn the disk consistently at the expected speed. Since the ability to
differentiate between wide and narrow bars in a code is related to the
speed at which the disk turns, it is critical that the motorized disk
continue to operate in a predictable way. Spinning speed will also relate
to Read Velocity, and may need to be adjusted to match the average speed
that a clerk will use to drag items through a supermarket checkout.
Mechanical failures may indicate a mismounted or imbalanced disk or other
mechanical problems that need to be corrected.

Optically, scanners are tested for code reading consistency. For a good
bar code scanner, this number should be greater than 85 percent. Commonly,
75 percent to 85 percent is achieved. If the scanner cannot meet this
criteria, it is sent back for an inspection of the optical
system—cleanliness of components and proper functioning of the
laser and detection system.

Electrically, scanners are tested for the Rejection Rate. Holographic
scanners scan the light over a bar code 100-200 times per second. This
allows the computer to compare many different readings of the code for
accuracy. But if there is some problem with the electronics, the computer
will begin to "reject" scans, or simply refuse to read them.
Part of this test uses bar codes that are imperfect in some
way—codes containing ink spots, bars of non-uniform width, etc. The
manufacturer has to produce a scanner that can tolerate some glitches in
the code printing process. This is another reason to use a multiple scan
and cross-check technique.

The Future

The future of bar code scanning technology will take a number of diverging
pathways. More general use of bar code scanning requires cheaper and
smaller light sources that will improve simple instruments like the wand
scanner. Semiconductor lasers, for instance, may make the wand a more
attractive instrument to users. In addition, some children's
learning tools and toys are starting to appear with interactive bar codes
rather than push buttons. In this way, new modules can be added to the
same bar code scanning toy. There are some home-shopping systems that are
beginning to exploit this technology, allowing people to do grocery or
clothes shopping at home by scanning selections from a catalog using their
telephone and a modem.

Laser scanners, on the other hand, are beginning to find more and more
complex applications as the technology becomes more reliable and easier to
use. More industries are using bar coding to track complicated lots of
custom-manufactured items, record steps
in a manufacturing process, and monitor activities in their plants. Other
optical assemblies may be developed that will allow this technology to
become even more flexible in size and utility.